Low-frequency backscatter radar systems, operating in the MF, HF, VHF, and UHF bands, are widely used for mapping and monitoring water surface targets such as currents, vessels, and waves on the ocean, or water flow along rivers. Nearly 150 such HF/VHF radars presently operate along the U.S. coasts as part of the U.S. Integrated Ocean Observing System (IOOS) program of the National Oceanic and Atmospheric Administration (NOAA), and such systems output their data to public websites (hfradar.ndbc.noaa.gov). Several other countries now have such radar networks on their coasts. A total of at least 400 of these oceanographic radars are deployed and operate worldwide.
At least two backscatter radars are normally needed to map currents, because each radar measures only a scalar radial vector component, and a view from two directions is needed to construct a total 2D vector for a map. These scalar velocities are based on the Doppler principle that separates the known Bragg-wave velocity from the unknown current velocity. In the case of a vessel target, its position and radial velocity are measured by a single radar, but a view from two radars offers the advantage of increased detection robustness.
Range or distance to the target or scattering cell is obtained from the time delay between transmit and received echoes, as is the case in all radars. Oceanographic radars in operation today employ FMCW signals (frequency modulated continuous wave), and commonly assigned U.S. Pat. No. 5,361,072, filed Feb. 28, 1992, entitled “Gated FMCW DF radar and signal processing for range/doppler/angle determination”, which is incorporated herein by reference, reveals how target range is derived from these signals. Following range processing, the complex (real and imaginary) echo time series for each range cell is Fourier transformed to obtain Doppler spectra and/or cross spectra among several receive antennas or elements. The velocity of the echoing target (current or vessel), as well as its bearing, is extracted from the signals at this point. One suitable and widely used bearing determination algorithm is Multiple Signal Classification (MUSIC), a direction-finding (DF) technique described in commonly assigned U.S. Pat. No. 5,990,834, filed Aug. 29, 1997, entitled “Radar angle determination with MUSIC direction finding”, which is incorporated herein by reference. This backscatter radar makes its measurements in a polar coordinate system in which radial current velocity at each point in the coverage area is measured by each radar on the polar map.
Because a single radar measures a single radial vector component in polar coordinates, normally two backscatter radar systems are used in pairs, spaced tens of kilometers apart and operating independently. Based on the known geometry and location of a mutually observed scattering cell, two resulting radial velocity components are combined to produce a total velocity vector map across the overlapping coverage zone. Thus, one shortcoming of conventional systems is the need for multiple, costly backscatter radar systems for current mapping as well as robust vessel surveillance.
In networks of coastal radars, greater data coverage and robustness for a given number of backscatter radars can be obtained by synchronizing these systems to a stable timing base and operating them multi-statically. The methodology for this is discussed in commonly assigned U.S. Pat. No. 6,774,837, filed Oct. 27, 2003, entitled “Ocean surface current mapping with bistatic HF radar”, which is incorporated herein by reference. The transmitter of one backscatter radar illuminates the sea surface, for example, where it is scattered by the waves or vessel target, and returns as echo to a different backscatter receiver. While thusly operating bistatically, each radar continues simultaneously receiving echoes in its normal backscatter mode. A convenient and inexpensive multi-static synchronization method in common use employs the stable time base of GPS satellite signals; this technique time-multiplexes the start times of each radar's FMCW modulation sweep in a controlled manner in order that their target echoes are distinctly and efficiently separated after demodulation so that they do not interfere with each other. This is discussed in commonly assigned U.S. Pat. No. 6,856,276, filed Mar. 28, 2002, entitled “Multi-station HF FMCW radar frequency sharing with GPS time modulation multiplexing,” which is incorporated herein by reference.
There are peculiarities and asymmetries of bistatic radar pairs within this multi-static configuration. For one, echoes with constant time delay behind the transmitter-receiver signal do not fall on circles as they do in backscatter radars. They fall on ellipses with the transmitter and receiver as the focal points. The scalar data from this pair occur in an elliptical coordinate system rather than the polar coordinate system of backscatter radars. Moreover, using this multi-static configuration, bearing with oceanographic radars is still measured at the receiving antenna, which is configured to estimate the angle to the echo. The transmitter is omnidirectional in its radiation, floodlighting the coverage area. This is a dissymmetry that favors the receiver end of the ellipse in terms of data quantity, quality, and robustness.
In a coastal network comprised of N backscatter radars with mutually overlapping target coverage, when operating multi-statically and measuring echo distance from conventional time delay, the target can be seen
            (              N        +        1            )        ·    N    2times, based on the conventional practice described in the above-noted U.S. Pat. No. 6,774,837. This compares with just N if the radars operated in the conventional monostatic (backscatter) mode. In the limit of large N, prior conventional multi-static operation provides N2/2 target measurements.
Accordingly, there is a need for improvement in expanding the number of measurements from the conventional multi-static operation discussed above, and removing the dissymmetry favoring the receiver end of the bistatic pair geometry.